Plasma treatments are frequently used to deposit thin films on and modify the surface properties of substrates used in a variety of applications including, but not limited to, integrated circuits, electronic packages, printed circuit boards, and medical devices. In particular, plasma treatment systems may be used to deposit various types of thin film materials onto substrates, such as optical and bio-medical coatings, insulating layers, polymers, and the like. Plasma treatment may also be used to prepare semiconductor and circuit board surfaces for electronics packaging. For example, plasma treatments may be used to etch resin and/or photoresist, to remove drill smear, to increase surface activation and/or surface cleanliness for eliminating delamination and bond failures, to improve wire bond strength, to ensure void free underfilling of chips attached to printed circuit boards, to remove oxides from surfaces, to enhance die attach, and to improve adhesion for chip encapsulation.
In a conventional plasma processing system, multiple substrates are placed inside a vacuum chamber between one or more pairs of electrodes. The vacuum chamber is then evacuated and filled with a partial pressure of a processing gas. Once the chamber atmosphere has the desired processing gas composition and pressure, energy is introduced into the chamber by exciting the one or more electrode pairs so that an electromagnetic field is generated between the electrodes. Each electrode pair is excited with a signal having sufficient energy to produce an electromagnetic field that at least partially ionizes the process gases, thereby generating a plasma. If an etching process is to be performed, the processing gasses and energy levels are adjusted so that the kinetic and chemical properties of the plasma result in the outermost surface layer(s) of atoms being removed from each substrate by physical sputtering, chemically-assisted sputtering, and chemical reactions promoted by the plasma. The physical or chemical action may be used to condition the surface to improve properties such as adhesion, to selectively remove an extraneous surface layer, or to clean undesired contaminants from the substrate's surface.
Plasma assisted film deposition methods typically operate by either chemical vapor deposition (CVD) or polymerization. If a CVD process is to be performed, the processing gas will include at least one precursor of the material being deposited. The precursor molecules are decomposed by the plasma formation process and the resulting precursor ions condense and react on the substrate surface to form a thin layer of the desired material. In some CVD applications, the substrate may also be heated to facilitate the deposition of the thin film material. If a polymerization process is to be performed, the process gas will include one or more monomer molecules. The monomer molecules are decomposed in the process of forming the plasma, forming ionized molecules that combine—or polymerize—as they condense on the substrate. The plasma initiated reaction of monomer molecules may thereby form a thin layer of polymer chains and/or three-dimensional networks on the substrate surface. Both the CVD and polymer processes may be used to produce thin conformal coatings on a wide variety of products.
Plasma processing systems including multiple electrode pairs allow both sides of multiple panels to be treated simultaneously in a batch process, which improves production throughput. To this end, a substrate holder locates each panel with a vertical orientation between pairs of planar vertical electrodes arranged in a rack, so that the environment between each planar vertical electrode and the adjacent surface of the panel provides a local process chamber in which the partially ionized processing gas—or plasma—is present. To generate the plasma, the electrode pair is energized by a power source with a suitable atmosphere present in the treatment chamber of the plasma processing system. Plasma processing systems employ power sources producing signals at various frequencies, with two commonly used frequencies being 40 kHz and 13.56 MHz. The frequency used to generate the plasma may affect both the chemistry of the plasma and how the plasma interacts with the substrate being treated. Deposition rates, as well as the quality and type of films deposited on the substrate may therefore vary with the frequency and intensity of the signal used to excite the plasma. For polymer film depositions, plasmas generated with higher frequency signals have typically been found to result in plasmas with improved chemistry that result in higher deposition rates and better quality films.
The panels processed in multiple electrode plasma processing systems may be quite large. For example, the panels may have a rectangular perimeter that is characterized by a width of about 26 inches and a length of about 32 inches. Electrodes must have an area at least as large as the panels being treated and the electrode rack may include a dozen or more electrodes spaced apart horizontally. The overall dimensions of the electrode rack may therefore be on the order of two to three feet in each dimension, thus requiring an equally large electrode excitation signal distribution system. As the dimensions of the electrodes and electrode racks increase, maintaining field strength uniformity across the entire surface area of each treated substrate as well as between substrates becomes more challenging. The problem of maintaining plasma uniformity may be exacerbated at higher electrode excitation frequencies because the dimensions of the electrodes and the excitation signal distribution system become a larger fraction of the excitation signal wavelength. Conventional RF bussing systems that achieve sufficient field uniformity at 40 kHz provide insufficient uniformity in multiple electrode plasma systems operating at higher plasma excitation frequencies such as 13.56 MHz. In addition, the input impedances of conventional RF bussing systems are difficult to match at these higher frequencies, resulting in high standing wave ratios and wasted RF power.
Therefore, there is a need for plasma treatment systems and methods to more uniformly distribute RF power to multiple electrodes in a plasma treatment system with improved input impedances at higher operating frequencies, such as 13.56 MHz.